=== App Tactic ===
The app tactic searches for functions and values
that matches each other, such that a new expression can be formed.

For example:

  use app
  f : (a -> b)
  x : a

The app tactic will suggest the following:

  Suggestions:
  0. ((f ↞ x) : b)

Type `0` to add the suggested expression.

  > 0
  (f : (a → b))
  (x : a)
  ((f ↞ x) : b)
